how to make each pixel the same setting color in the rendered image?
If it is an identification of the rendered part you want (where every object has its own colour), use the ID render pass. In the object properties, set the "Pass Index" to a unique number. In the Render Layer properties, enable the Object Index pass.



Then use the IndexOB slot of the Render Layer node to render the colours. For each pixel, this slot will give you the value of the "Pass Index" property you set on the object. You can use a ColorRamp node to map from a number to a colour; however, that accepts only numbers between 0 and 1, so you have to divide the IndexOB by the highest number you used. In my example the background has index 0, the cube has index 1, and the sphere has index 2, so I divide by 2.